Microelectromechanical system (MEMS) devices are have become established as useful commercial items. One problem that can arise in MEMS devices is that the sizes of micromachined parts are generally comparable with the sizes of airborne dust particles and so the operation of micromachines can be perturbed by dust. In particular, the operation of MEMS devices is susceptible to leakage currents conducted by particles as well as to mechanical interference. Generally speaking, the higher the electrical impedance of such systems, the higher is their vulnerability. This is particularly true for systems that are electrostatically driven and capacitively sensed, such as the majority of MEMS gyroscopes and accelerometers in mass production today.
In order to maintain high quality and reliability, the semiconductor processes used for micromachining are conducted in scrupulously clean conditions. Also, it is desirable to design the MEMS device in such a way that it is as immune as possible to such particles. In reality, perfect immunity, like perfect cleanliness, is impossible to achieve and must be supplemented. The additional measures include protection such as capping, for example, as described in U.S. Pat. Nos. 6,323,550, 7,416,984, and 7,458,263, each of which is hereby incorporated by reference in its entirety, and optical inspection or electrical test methods which detect particles so that MEMS parts containing them can be identified.
The concepts of immunity to particles and detectability by electrical test are generally at odds with one another. Generally speaking, the more immune a structure is to perturbation, the less easy it is to perceive the presence of a particle, while sensitive structures which disclose particles easily are generally not immune.